A septum magnet is an apparatus that generates a magnetic field in the duct, which is provided in such a way as to share a tangential line with a circulation orbit for a particle beam, for the purpose of making a particle beam in a duct move onto the circulation orbit or making a particle beam existing on the circulation orbit be taken into the duct. In a basic septum magnet, in a gap of a yoke that has a C-shaped cross section, whose opening is oriented to the outer circumference thereof, and that extends in the shape of an arc, a duct that extends in the shape of an arc is disposed in such a way as to be inserted between a septum coil at the outer circumference thereof and a return coil at the inner circumference thereof. The septum coil and the return coil are connected in series with each other in such a way that currents having the same intensity but the opposite directions flow in the circumferential direction. As a result, a magnetic field perpendicular to the circumferential direction, which is the traveling direction of a particle beam, and perpendicular to the radial direction, is generated in the duct, so that the particle beam can be deflected in the radial direction.
Although referred to as coils, the septum coil and the return coil are each formed of copper pipes that are coupled end to end so as to become coil-shaped, because they need to be cooled; therefore, unlike a typical winding coil, they have a high rigidity. Because strong force is exerted on the coil thereof during its operation, the septum magnet is configured, for the convenience of maintenance, in such a way that the yoke thereof can be separated upward and downward in the axis direction, i.e., in the vertical direction at a time when the septum magnet is installed. In this situation, each of the septum coil and the return coil, which have a high rigidity, needs to be separated upward and downward along with the yoke. In this case, the upper coil and the lower coil needs to be separately positioned in such a way that the upper septum and return coils are fixed to the upper yoke and the lower septum and return coils are fixed to the lower yoke. Thus, when the positions of the upper coil and the lower coil differ from each other in the radial direction, an unnecessary radial-direction magnetic field (a skewed magnetic field) is generated.
Accordingly, for example, it is conceivable that a skewed magnetic field is suppressed by use of a technology in which an auxiliary coil is provided in the vicinity of the region where the distribution of the magnetic field needs to be improved (e.g., refer to Patent Documents 1 through 4).